changing the gearing on my cross bike for the three peaks and want to know if i can run an xt 10 speed cassette with my ultegra 10 speed shifters if so which rear mech will i need.ta

A 10 speed MTB cassette is 1mm wider than a road one so your road shifter won't index properly with it.

I just swapped a 10 speed 105? road cassette for a new 10 speed XT on our road tandem, with 105 STI shifters, 105 front mech, truvativ road triple cranks and 9 speed XT rear mech. It works OK for us, sometimes mis-shifts changing up a gear but that could just be cable stretch, tandems have looooong cables

strange,why would shimano manufacture two ten speed groupsets with 1mm difference?The other option is buy the shifters and run a flat bar i suppose

I don't know why they would do it but that is what they have done. I would imagine it is something to do with performance in dirty conditions.

Are you sure about this Mister P? I thought quite a few people ran road cassettes on MTBs. For instance doesn't Steve Peat and probably a few other DH riders?

BenW – Wiggle seem to have the new Sram Apex cassettes listed so you could use one of those. 10 speed and I think go to a 32 and a 34 lowest sprocket.

Does 1mm make that much difference? What's the float on an upper jockey wheel anyhow?

Thanks for that 1961Bikie i had never even heard of the groupset

10 speed road casettes come with a 1mm spacer that sits behind them on the freehub.. **** only knows why…
Prob the MTB ones don't need that spacer, but still have the same spacing from one sprocket to the next.

Benw, just been through the same thing, but due to knackered STI's was starting from scratch. I've gone for SRAM STI's, a compact chainset, an Apex 11-32 cassette and matching long cage mech'. This set up works a treat and gives me a super low (by cx standards) gear. Ive been testing it on our local hills, the Cairngorms, and I'm pedalling up some pretty big climbs. Coming from Shimano STI's the double tap systems taking some getting used to, but I really like the mtb feel to the shift – SRAM on the mtb.
Now just need to "up" the training, slight panic setting in!

Oh, and there seems to be more of these 3Peaks gear threads popping up now. We all getting a bit worried!

Now then I've got a 10 speed XT cassette in the dining room and a ten speed road bike somewhere round the house – can I be bothered offering tout.he two cassettes up.

If there's not definitive answer soon give me a shout

PS – why not run smaller chainrings and leave the cassette alone?